Reenable some supressed warnings for the objc SDK.

Bug: webrtc:8441
Change-Id: I6b427dfc1fe275e274d042766e0850628cf19994
Reviewed-on: https://webrtc-review.googlesource.com/15000
Reviewed-by: Anders Carlsson <andersc@webrtc.org>
Reviewed-by: Henrik Andreassson <henrika@webrtc.org>
Commit-Queue: Kári Helgason <kthelgason@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20425}
diff --git a/sdk/BUILD.gn b/sdk/BUILD.gn
index d415158..23f626f 100644
--- a/sdk/BUILD.gn
+++ b/sdk/BUILD.gn
@@ -42,6 +42,10 @@
       "objc/Framework/Classes/VideoToolbox",
       "objc/Framework/Headers",
     ]
+    cflags = [
+      "-Wstrict-overflow",
+      "-Wmissing-field-initializers",
+    ]
   }
 
   rtc_static_library("common_objc") {